fix -Wunused-result warnings when calling write()

Fix these warnings:

papertrail_proc.hh: In constructor ‘papertrail_proc::papertrail_proc(const string&, time_t, time_t)’:
papertrail_proc.hh:107:36: warning: ignoring return value of ‘int asprintf(char**, const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
                  this->ptp_api_key);
                                    ^
papertrail_proc.hh: In member function ‘void papertrail_proc::set_url()’:
papertrail_proc.hh:146:47: warning: ignoring return value of ‘int asprintf(char**, const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
                  this->ptp_quoted_search.in());